A known spindle drive (DE 20 2008 016 615 U1) on which the invention is based is generally provided with a feed gear mechanism consisting of a spindle and spindle nut, a drive motor being associated with the spindle nut.
Overall, the spindle drive is divided into a drive portion on the spindle side and a drive portion on the spindle nut side. The drive portion on the spindle side carries the drive motor. Actuation of the drive motor leads to a linear, relative adjustment of the two drive portions to one another. In this case, tubular housing parts, which interlock in a telescopic manner, are associated with the two drive portions.
For the coupling to the adjustment element, on the one hand, and the bodywork of the motor vehicle, on the other hand, in each case a ball cup is associated with the two drive portions, which in each case cooperate with a ball arranged on the adjustment element and, together with the respective ball, form a coupling means. In this case, the ball cup of the drive portion on the spindle nut side is connected to the spindle nut via a connecting tube.
It is particularly advantageous in the known spindle drive that a spring arrangement is provided between the two drive portions which pretensions the two drive portions in the extended position. Thus a compensation for the weight of the adjustment element may be achieved in an elegant manner.
The pretensioning force of the spring arrangement may, for example, be approximately 1000 N. For security, the drive train associated with the spring portion is generally designed so that it may withstand a tensile force of at least 5000 N. This represents a specific requirement for the structural design, as the corresponding part of the drive train generally contains force-transmitting stamped connections or the like, which lead per se to a certain weakness of the drive train.
In some cases, even the aforementioned 5000 N are not sufficient in order to prevent the spindle drive from violently falling apart. This is the case, for example, if the adjustment element is accelerated manually in an extreme manner, so that an extreme tensile load acts on the spindle drive on both ball cups. The spring arrangement, as a result, is released abruptly. The resulting complete relaxation of the spring arrangement also takes place abruptly, as a result of the extreme pretensioning, and is associated with a considerable risk of injury to the user. Therefore, it has already been proposed to design the part of the drive train associated with the spring arrangement to be even stronger, which however is associated with considerable additional costs.